Background: The anion gap (AG) is an important tool in the evaluation of metabolic acidosis. It is affected by many variables including serum albumin and globulin concentrations. HIV patients may have lower serum albumin and higher serum globulin concentrations. We hypothesized that the AG in HIV patients may differ from that of normal controls.
Patients and methods: We reviewed medical records of 248 stable HIV patients and compared their laboratory variables to 312 patients being evaluated for routine health maintenance in an outpatient setting.
Results: The average serum albumin concentration was not different in patients with HIV and normal controls (43 +/- 6 g/L vs. 45 +/- 4 g/L). The serum globulin concentration was significantly higher in the HIV patients when compared with that of normal controls (37 +/- 9 g/L vs. 28 +/- 6 g/L; p<0.05). The AG in the HIV patients was significantly lower than that of normal controls (9.4 +/- 1.9 mmol/L vs. 10.8 +/- 2.7 mmol/L; p<0.05). The slope of the regression line that describes the inverse relationship between serum globulin and AG was 0.147 mmol per g/L. Using this slope, AG could be adjusted for abnormal serum globulin levels: adjusted anion gap = anion gap + 0.147 x (globulin - 29).
Conclusion: Our results indicate that the AG is lower in HIV patients and that this decrement may be due to the increase in serum globulin concentrations. Since a high serum AG metabolic acidosis may be masked by a deceitfully normal AG in patients with elevated serum globulin concentrations, calculation of corrected AG should be undertaken to avoid a costly delay in diagnosis and treatment.